Not applicable.
Obtaining images of high aspect ratio structures poses certain challenges. High aspect ratio structures are those where one characteristic dimension is more pronounced than the others. Examples of these type of structures include probes of atomic force and scanning tunnelling microscopes, field emission probes, micro-indenters and Micro Electro-Mechanical systems (MEM""S) structures. Such high aspect ratio structures have typical heights on the order of tens of micrometers and tip radii in the range of tens of nanometers. Further, these structures may or may not be conductive.
In imaging such structures, it is sometimes desirable to image the side walls of the structure and to measure the radius of the tip of the structure in a non-destructive manner. Desired image resolutions can be on the order of 1 nm in the vertical (normal to the surface) direction and 10 nm in the lateral direction. Such imaging criteria prevent the use of certain types of profilometry methods.
In one type of conventional stylus profilometer, a stylus with a sharp tip is mechanically dragged along the sample surface. The deflection of the hinged stylus arm is measured and recorded as the surface profile. The use of a hinged stylus arm allows measurement of very rough surfaces, for example those with peak-to-peak heights greater than 1 mm. Probe-to-surface contact forces range from 10xe2x88x923 N to 10xe2x88x926 N. However, since the hinged stylus arm is partially supported by the stylus itself, physical rigidity limits the minimum stylus tip radius and hence the lateral resolution to about 0.1 mm.
In optical profilometry, many different optical phenomena (such as interference and internal reflection) can be utilized. The most popular technique is based on phase-measuring interferometry, in which a light beam reflecting off the sample surface is interfered with a phase-varied reference beam. The surface profile is deduced from the resulting fringe patterns. With a collimated light beam and a large photodetector array, the entire surface can be profiled simultaneously. This and other conventional optical profilometry methods are limited in lateral resolution by the minimum focussing spot size of about 0.5 xcexcm (for visible light). In addition, measurement values are dependent on the surface reflectivity of the material being profiled.
Currently, only the recently developed scanning probe microscopes can meet a 10 nm lateral resolution requirement. In these microscopes, an atomically sharp (or nearly so) tip at a very close spacing to the sample surface is moved over the surface using a piezoactuator. One type of scanning probe microscope is the atomic force microscope (AFM), which measures the topography of a surface with a probe that has a very sharp tip. A probe assembly includes a cantilever beam from which the probe, or microstylus extends. The probe terminates at the probe tip having a typical tip radius of less than 0.1 xcexcm. The probe typically has a length on the order of a couple of micrometers and the cantilever beam typically has a length between 100 xcexcm and 200 xcexcm.
As is illustrated in FIG. 1, the AFM can operate in two different regimes, contact and non-contact, depending on the spacing maintained between the probe and sample. In the contact regime, the probe is kept some angstroms from the sample surface and the interactions are mainly repulsive. In the non-contact regime, the spacing between the probe and the sample surface is from tens to hundreds of angstroms and the interactions are attractive, mainly due to the long range van der Waals forces.
In a contact mode atomic force microscope, the probe is moved relative to the surface of a sample and deflection of the cantilever is measured to provide a measure of the surface topography. More particularly, a laser beam is directed toward, and reflects off the back surface of the cantilever to impinge upon a sensor, such as a photodetector array. The electrical output signals of the photodetector array provide a topographical image of the sample surface and, further, provide feedback signals to a fine motion actuator, sometimes provided in the form of a piezoelectric actuator. In a constant force contact AFM, the fine motion actuator is responsive to the feedback signals for maintaining a substantially constant force between the probe tip and the sample, such as forces on the order of 10xe2x88x928 N to 10xe2x88x9211 N.
Initial contact between the probe and the sample is conventionally achieved with the assistance of a camera located above the sample. The probe and sample are visualized with the camera and, once the probe is positioned at a desired area of the sample, the user actuates a coarse motion actuator which moves the probe into contact with the sample surface. Generally, the coarse motion actuator has a relatively large vertical range, such as on the order of 2-10 centimeters.
Contact atomic force microscopy offers high lateral and vertical resolutions, such as less than 1 nm vertical resolution and less than 50 nm lateral resolution. Further, since the contact AFM relies on contact forces rather than on magnetic or electric surface effects, advantageously the contact AFM can be used to profile conductive and non-conductive samples. However, the maximum surface roughness that can be profiled is much less than that of conventional stylus profilometers which use a linear variable differential transducer (LVDT).
In the non-contact atomic force microscope, long range van der Waals forces are measured by vibrating the cantilever near its resonance frequency and detecting the change in the vibrational amplitude of a laser beam reflected off the cantilever due to a change in the force gradient caused by changes in the surface profile. The non-contact atomic force microscope offers non-invasive profiling. However, the technique has some disadvantages when compared to contact atomic force microscopy. First, van der Waals forces are hard-to-measure weak forces, rendering the microscope more susceptible to noise. Secondly, the probe tip must be maintained at a fixed height above the sample, typically on the order of a few nanometers, and the feedback control necessary to maintain this spacing must operate slowly to avoid crashing the probe tip on the sample. Thirdly, since the tip is always floating above the surface, the effective tip radius is increased and hence the achievable lateral resolution is decreased.
According to the invention, methods and apparatus utilizing contact atomic force microscopy are provided for profiling both conductive and non-conductive samples having high aspect ratio structures, with a lateral resolution on the order of 10 nm and a vertical resolution on the order of 1 nm. Conventional atomic force microscopes are typically used to provide a topographical image of relatively flat surfaces and certain problems arise when using atomic force microscopy to profile high aspect ratio features. As a result, high aspect ratio structures are most typically imaged under a Scanning Electron Microscope (SEM). However, AFM imaging is more desirable because the topographic data retrieved is already in numerical format, whereas SEM pictures must be interpreted, based on the image contrast.
Various aspects of the present invention address and overcome the problems faced when using an AFM for profiling high aspect ratio structures. In conventional AFMs, a user controllable coarse motion actuator is used to bring the probe into initial contact with a desired area of the sample surface with the assistance of a camera. Although the camera facilitates landing the probe in a desired area of the sample, due to the extremely small dimensions of the probe tip and high aspect ratio features and also due to the practical resolution limitations of the camera, this technique is not generally capable of reliably landing the probe tip on a high aspect ratio feature to be profiled. If the probe tip initially lands at the base of a feature, the fine motion actuator used to scan the sample during image acquisition may not have sufficient vertical range to permit the probe tip to climb from the base of a feature to its apex. Further, such climbing induces undesirable wear on the probe.
The probe landing techniques described herein are used to ensure that initial contact of the probe is with the apex region of a high aspect ratio feature to be profiled. This is achieved by performing scanning steps prior to bringing the probe into contact with the sample. More particularly, the described probe landing techniques include moving the probe relative to the feature until the probe tip is in close proximity to the feature, moving the probe in a first scanning pattern to locate the feature, and then moving the probe in a second scanning pattern to image the feature once the feature is located.
One embodiment of the invention, referred to as the cantilever landing technique, includes the additional step of landing the cantilever of the probe assembly on the feature prior to moving the probe in the first scanning pattern. Once the cantilever contacts the feature, the probe scans xe2x80x9cbackwardsxe2x80x9d to locate the feature.
In another embodiment, referred to as the progressive approach and engagement technique, movement of the probe in the first scanning pattern includes initially moving the probe in a scanning pattern in air. The probe is progressively lowered to scan in lower and lower horizontal planes until contact of the probe with the feature is detected.
The probe tip is brought into close proximity with the sample feature using at least one camera. For example, in the cantilever landing embodiment, the probe is brought into close proximity with the feature by visualizing the cantilever and feature and moving the probe relative to the feature until the cantilever and feature are in intersecting vertical alignment. Since the cantilever beam contacts the feature, the resolution provided by a top view of the sample is sufficient to achieve the benefits of the cantilever landing technique. In the progressive approach and engagement technique, preferably, the AFM includes a second camera laterally positioned with respect to the sample which is used in conjunction with the vertically positioned camera in order to more closely locate the probe tip relative to the feature prior to scanning.
In accordance with a further aspect of the invention, a deconvolution strategy is provided. Deconvolution is necessary in applications in which the sample feature dimensions are in the same range as the size of the probe tip, therefore resulting in strong image convolution. The deconvolution technique includes scanning a standard sample with a probe to provide a mean curve for the probe shape, an inner curve for the probe shape, and an outer curve for the probe shape, thereby characterizing the probe shape. The standard sample is a physical sample for which a theoretical description is provided including a curve of the sample""s mean profile and curves of the inner and outer bounds of the sample""s profile. The theoretical curves are compared to the scanned image of the standard sample to provide the inner, outer, and mean curves for the probe shape.
Thereafter, a sample to be profiled is scanned with the probe to provide an image of the sample. The sample image is then deconvolved using the mean curve for the probe shape, the inner curve for the probe shape, and the outer curve for the probe shape to provide a mean curve for the sample, an inner curve for the sample, and an outer curve for the sample.
Also described is a technique for measuring the tip radius of high aspect ratio features which includes selecting points along the apex of the image, fitting a circle to the points such as with the use of a progressive least square technique, and measuring the error associated with the circle. If the measured error is smaller than a predetermined error, then at least one additional point along the apex of the image is selected and the preceding steps are repeated. Alternatively, if the measured error is greater than the predetermined error, then the best fit has been found and the radius of the circle is provided as the radius of the profiled feature. Illustrative measures of error include a maximum individual error between a point on a fitted circle and the feature image (i.e., a Maximum Fitting Error, or MFE) and a cumulative error between each of the selected points on the fitted circle and the feature image (i.e., a Maximum Distributed Error, or MDE). In one embodiment, both the MFE and MDE are used to measure the tip radius.